Provided that EC2 and RDS are on the same VPC, is there an internal endpoint we need to use to speed up data exchange between the two?
Similar to how if MySQL + Apache are on 1 server we would connect using 'localhost' (avoiding TCP/IP).
EC2 and RDS are not running on the same server, so localhost would not work. A VPC is still a network, and you would still use TCP inside that network to connect from one server to the other.